Why are temperatures on the continent soaring?

The heatwave has already caused flash floods and forest fires, grounded planes and buckled train tracks and led to school closures and warnings over air quality. France has set up temporary water fountains in built-up areas and is allowing public pools to open late into the night. In Germany, people are filling hot water bottles, placing them in freezers and then taking them to bed.
 As the mercury continues to rise - with potentially deadly consequences - experts tell the BBC what is behind the heatwave.